So lets look at a situation. You're playing in a game, and your warrior is one of the finest in all the land. You've been developing him over the last year or so of game, and he is finally starting to get to where you want him to be. You get into a fight with an NPC, another in what is quickly becoming a long line of challengers. Only the NPC does something dirty, he blinds you with sand, and then permanently with a strike of the blade. You've lost your sight, you lose the fight. The character is ruined. Worse, the character wasn't killed, he is still alive, sitting their, but he can't do what you want him for now. You ask the GM if you can get your eyes fixed, they say no. Now you're angry about it, why would the GM take the character away? Especially after all this time working on it.
So now, on the other side of the screen you're the GM. You had this planned, an obstacle for the character to give the player something they've asked about time and again int he time you've known them. They love the idea of a Blind Swordsman, so you decide to help them get it. You blind the character, the eyes can't be fixed. The plan is to have the character grow and develop blind fighting and come back even better than they are now. So why is the player mad at you? I mean, 1) it's just a game, and 2) something really cool is going to come out of this.
Do you see the problem? The problem is the GM knows where things are going, but the player doesn't. Not many people are able to just ride it out through thick and thin without a word of complaint or emotional reaction. Negative emotions are also easier to stir up from a tense situation than positive ones, and if the Player really liked the character well...maiming is a lot more brutal than killing. At least killing is an end, there is a fair and impartial rule in the system for death. There really isn't (in most systems) for permanent blinding. So not only is the Player's character blind, but quite literally the GM did it to them.
Even with trust the situation can be annoying, and will lead to heated words. Trust between the Player and GM can make it work still, but more likely the trust is going to be broken. It feels like an attack, attacks break trust. So what do you do? Well, as I've said dozens of times here before, you communicate. This entire problem (in the example) could be avoided if the GM simply went "So, you keep saying you want a Blind Swordsman, how about we do it with your new guy? I'll blind him sometime in the future and then he can become one". The player doesn't know the when or how, but they know the what. If they are totally against the idea, they can say no and you are both saved a lot of hassle. If they like it, they can say yes and while they still don't know all the details, when it does happen they know that it is part of a plan to give them something cool. They won't have that angry flip out moment. What is even better though is that fights will become more tense for them. Is this the time that they will be blinded? Is today the last sunset their character will see? They know the blinding is coming, but they don't know when, the added tension can be a nice additional effect.
See, the thing is that sometimes when we're GMing we get so caught up in how awesome we think something will be that we forget to consider if it is something the players will also think is awesome, or just something we think is awesome. The Player may love blind swordsmen, but that doesn't necessarily mean he wants his current character to be one.
At the same time, as Players there is a tendency of seeing when things go drastically wrong as the GM attacking you, even if they are trying to set up drama. We forget, however temporarily, that the game is to tell a story, and that stories involve low moments as well as high. We get caught up in the pain our characters are feeling, as well as the pain of having a favored toy broken and we lash out.
The answer to both of these is communication. Forewarned there is much less getting caught up in the moment Out Of Character and feeling angry about it. You knew it was coming, so you are mentally prepared. Even better, you know that something good is going to come out of it in the end, which means you can just enjoy it. You know it is not the end of the character's story, just the beginning of another chapter and that can be all that is needed to make it easier to endure.
So talk to each other. You don't have to give out all the details, but the main gist of what you have planned is good. Even if it is something as simple as "Man, this next session is going to be really rough, don't forget to bring your thinking hats" it can stop a bad situation from arising as the players feel you are lumping things onto them too heavily. As players you need to talk as well, if the GM is proposing something you really don't like or want, you need to tell them. Otherwise you have no one to blame but yourself when it happens.
So talk to each other, and remember...